Animal tissue

“ In the adult animal the body is composed of two kinds of tissues, those which are connected with or at all events are under the control of the nervous system, and those which are capable of leading a free life independent of the nervous system. These two kinds of tissues can be traced back from the adult to the embryo, and it is the task of embryology to find out how these two kinds of tissue originate. ”

“ I remove, for the length of two or three centimetres, the skin from the tip of a young rat's tail, and insert the flayed part into the subcutaneous cellular tissue through an opening made in the skin of the animal's back. A few stitches suffice to hold the parts in place, and soon they adhere firmly, the rat's tail then having the ansate form. ”

“ It seems probable, however, that these cells, separate as they seem, are in some cases united to each other by minute protoplasmic filaments. In the higher animals the cells in different parts of the body take on different forms and perform different functions. Like cells with like functions are also aggregated together into tissues. Thus the surfaces of the body, external and internal, are bounded by or lined with epithelial tissue; the bones and framework of the body are composed of skeletal tissue; nervous tissue goes to form the brain and nerves; contractile tissue is found in the muscles ”

“ Out of the blood all of the tissues are wrought, each in its kind, and every tissue is a kind of its own, and there are kinds of kinds, so that the animal organism is a chemical laboratory engaged during the existence of the animal in building up more complex substances and tearing them down into more specialized substances, and this is metabolism, or zoochemistry. ”

“ The blood must be deprived of its cells by the centrifugal process and must generally be taken from the animal for which the tissue is to be cultivated or, at any rate, generally from the same species, although this is not without exceptions, for chicken tissue has been grown in the blood of human beings, dogs, and rabbits; morbid tissue, perhaps, like cancer, being most indifferent. The tissue is taken from an etherized subject, with every possible precaution against bacteria, chilling, or drying, and so liable is it to be killed by exposure to air that it is best dissected in serum. ”

“ Preparation of Animal Tissues.—Animal tissues differ from one another so greatly, both in consistence and in chemical composition, as well as in their degree of natural hardness, that no general rules can be given which would be applicable to the preparation of the whole class. Such as are of any considerable degree of hardness, as horn and kindred structures, must be treated much in the same manner as the denser varieties of wood, viz., by more or less prolonged immersion in water—cold, hot, or boiling. ”

“ If we take a slice of the bone and examine it, we shall find that it is very like this diagram of a section of the bone of an ostrich, though differing, of course, in some details; and if we take any part whatsoever of the tissue, and examine it, we shall find it all has a minute structure, visible only under the microscope. All these parts constitute microscopic anatomy or 'Histology.' These parts are constantly being changed; every part is constantly growing, decaying, and being replaced during the life of the animal. The tissue is constantly replaced by new material ”

“ In the whole scheme of evolution we can recognize, not only an upward progress in the organization of the animal as a whole, but also a distinct advance in the structure of the tissues composing an individual, which accompanies that upward progress. Thus it is possible to speak of an evolution of the supporting tissues from the simplest form of connective tissue up to cartilage and thence to bone ”

“ So much of the tissues as are necessary for the experiments to be performed are taken, and to them is added at most five times their quantity of water. If any difficulty arises in the boiling, which may be traced to the insufficient quantity of tissue used, then more tissue is added, the excess of the organ being immediately put back into the bottle that contains the rest, in case it may be wanted later on. ”

“ The materials which compose animal bodies are endowed with properties which differ in every different tissue: for instance, we say that the muscular tissue has the property of contracting. These properties of tissues develop functions: for example, the contraction of the ciliary muscle permits correct vision; but it is evident that if the tissue does not keep up the process of nutrition, i. ”

“ The texture of the body, in which all these varieties have their seat, is the extracorial or exodermal structure, constituting, if I may so speak, the outer coating of the body, external to the true skin, which corresponds to the cuticular and corneous excrescences of animals—a structure which includes horns, hoofs, hair, feathers, and all similar appendages in different orders of animals. This structure displays infinite diversities in colour, constitution, and organization, and is the most variable tissue on the whole body. ”

“ Of the Composition of the Tissues and of the Secretions in Health and Disease.—Having described thus individually the constituents of the tissues, we shall now present such results as have been hitherto obtained as to the quantitative composition of the organized tissues formed by their reunion, their secretory products and morbid alterations.
The skin of animals is a congeries of finely constructed organs, sensitive and secretory, imbedded in a peculiar tissue, which is one of those most yielding gelatine, whence the process of tanning skins. ”

“ As a matter of fact, the idea that parts of the tissues of different animals may serve to replace one another has been employed repeatedly in science, especially in the science of medicine. But I believe that our ideas are not yet clear upon the matter. The erroneous impression to which I have alluded has arisen because we do not bear in mind that each tissue, each part of an organ, each cell, possesses, in addition to its obvious characters, very many characters that are invisible to us. ”

“ A relevant inquiry suggests itself—What proportion {568}of species-marks are formed out of inanimate tissue or tissue of low vitality—tissue which, like hair, feathers, horns, teeth, is composed of by-products unfit for carrying on vital actions. ”

“ The reader will observe the common use throughout the whole animal kingdom of the skin unmodified. Also that the special organs of the invertebrates, which number more than nine tenths of the whole, are quite limited to modifications of the skin; while those of vertebrates belong only to the alimentary canal. The description to follow will adhere to the order of the table:
Respiration by the Unmodified Skin and Food-Tract.—The general surface of the body has in all soft-skinned animals an aërating or respiratory action ”

“ The outer or animal layer, or ectoderm, always forms the chief organs of animal life—the skin, nervous system, sense-organs, etc.; the inner or vegetal layer, or entoderm, gives rise to the chief organs of vegetative life—the organs of nourishment, digestion, blood-formation, etc. ”

“ DERMOID SYSTEM.
All animals are covered with a more or less compact membrane, of a thickness in general proportioned to the size of their body, destined to defend the subjacent parts, to carry out a considerable portion of the residue of nutrition and digestion, and to place it in relation with external bodies. It is in man a sensitive boundary, placed at the extremity of the domain of his mind, where these bodies continually touch, for the purpose of establishing the relations of his animal life, and of thus connecting his existence with that of every thing which surrounds him. ”

“ The general circulating fluid of animals, and that on which the nourishment and growth of their bodies depend, and from which all the secretions are formed. It is warm and red in vertebrated animals; and, for the most part, cold and white in the invertebrata. In man and all other mammals, and in birds—the two highest classes of the animal kingdom—the blood, though collectively forming but one circulating stream, varies considerably in appearance according to the part or vessels in which it is found. ”

“ These tracheæ are everywhere bathed by the blood, and thus the latter is constantly aërated or kept fresh; the blood not, as in vertebrates or as in molluscs, seeking the lungs or gills, or any specialized respiratory portion of the body where the oxygen combines with the hæmoglobin, but the respiratory tubes, so to speak, themselves seek out the blood and the blood-tissue in every part of the insect body, penetrating to the tips of the antennæ and of the legs, entering the most delicate tissues, even perhaps passing through the walls of epithelial cells. ”

“ Mix a wineglass of muriatic acid with a pint of water, and place in it a sheep's rib. In a day or two, the bone will become so soft that it can be tied into a knot. In the same way, an egg may be made so pliable that it can be crowded into a narrow- necked bottle, within which it will expand, and become an object of great curiosity to the uninitiated. By boiling bones at a high temperature, the animal matter separates in the form of gelatine. Dogs and cats extract the animal matter from the bones they eat. ”

“ These are highly complex substances, forming the basis of all animal and vegetable tissues. There are many different kinds of proteins, varying somewhat in their constitutions, but all show, on analysis the following approximate composition of chemical elements:
The principal kinds of proteins found in the various fur structures are albumins, keratin, collagen, and mucines. Albumins, of which the white of egg is the most familiar variety, occurs to some extent in the corium as serum in the blood-vessels, and also as the liquid filling the connective tissues, known as the lymph. ”

“ The result is the same, you see, although the organ is different; and, remarkably enough, these little tubes are wanting among fishes, which, like carp, have a species of salivary glands in their mouths, of which the others show no trace; from which one may fairly conclude that these glands and tubes mutually supply each other's places. Here, then, you see an instance of the light which different animal organisations throw upon each other when they are compared together. ”

“ The tissues of plants, epidermis, ducts, and woody fibres are constantly found in microscopic preparations. They may be studied in delicate sections made with a sharp knife, or by tearing vegetable tissues apart with needles. The down of moths, the hairs of different animals, the fibres of paper, the most common animalcules in water, the dust of shelves, and generally the structures found in all vegetable and animal substances by which we are surrounded, should be studied as a preliminary to any special line of microscopical investigation. ”

“ Scent-glands, &c.—Besides the universally distributed sweat-glands connected with the hair-system, most mammals have special glands in modified portions of the skin, often involuted to form a shallow recess or a deep sac with a narrow opening, situated in various parts of the surface of the body, and secreting odorous substances, by the aid of which individuals recognize one another. These probably afford the principal means by which wild animals are able to become aware of the presence of other members of the species, even at great distances. ”

“ No other animal gives nourishment to its young in such fashion; all mammals do. What is the origin of the habit? How did the organ arise?
A part of an animal's body that has the power to gather material from the blood and pour it out in the shape of fluid is known as a gland. Sometimes a whole organ does nothing else. Sometimes small glands are scattered through, or over, the surface of another organ. There are two kinds of glands in the skin of the mammal. The best known and most frequently thought of are those which pour out the perspiration. ”

“ If the stomach of an animal is organized so as to digest only flesh, then the jaws and the incisive teeth must be constructed for devouring flesh; the claws for seizing the prey; and the entire system of the limbs for pursuing and catching it. Every one of those organs is indispensable in the structure of carnivorous animals ”

“ All other parts of the efferent apparatus (uterus, vagina, receptaculum seminis, ductus ejaculatorius, penis, and appended glands) develop from the hypodermis.
3. The connective tissue and the musculature of the efferent apparatus are derived from mesoblast cells present in the body-cavity.
4. The efferent ducts originate as paired rudiments. All unpaired (azygos) parts (uterus, penis, receptaculum seminis, unpaired glands, etc.) are at first paired. The unpaired efferent apparatus of insects must therefore be regarded as morphologically a secondary and more complicated form. ”

“ BAD MEAT. In the flesh of animals slaughtered whilst suffering acute inflammation or fever, the hollow fibres, or capillaries, as they are called, which form the substance of the lyer, are filled with congested and unassimilated animal fluid, which, from its impurity, gives the lyer a dark colour, and produces a tendency to rapid putrefaction. In a more advanced stage of such disease, serous, and sometimes purulent matter, is formed in the cellular tissues between the muscles of the flesh ”

“ In the first place, this feather, as an anatomist would tell us, is "a dermal modification"—in other words, an altered bit of the skin. Every part of a plant or animal undergoes changes, our modern teachers say, just in accordance with the external influences which affect it. But the skin of an animal is naturally exposed to many more such surrounding agencies than its internal organs. ”